Name: conducting elevated temperature normal and combined pressure-shear plate impact experiments via a breech-end sabot heater system
Uploaded: 2018-08-07T12:30:00
Description: Watch this Scientific Journal Video about Conducting Elevated Temperature Normal and Combined Pressure-Shear Plate Impact Experiments Via a Breech-end Sabot Heater System at JoVE.com

The authors would like to acknowledge financial support of the U.S. Department of Energy through the Stewardship Science Academic Alliance DOE/NNSA (DE-NA0001989 and DE-NA0002919) in conducting this research. Finally, the authors would like to thank Los Alamos National Lab for their collaboration in support of undergoing efforts in the current and future investigations.

1. Sample and Target Material Preparation
NOTE: In the following protocol, we will detail the steps necessary for preparing the sample and target materials, which will be later used in a reverse geometry normal plate impact experiment. In this setup, a flyer plate (also the sample), held at the front of a sabot, will be launched via a single stage gas gun and made to impact a stationary target plate housed in the target chamber of the gas gun. A typical flyer and target plate assembly described ...

<ol>
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The method and protocol stated above detailed the procedure for properly performing a reverse geometry normal plate impact experiment at elevated temperatures. In this approach, we make custom modifications to the gun barrel at the high pressure (breech) end of the existing gas gun at Case Western Reserve University, to house a resistive heater coil with axial and rotational degrees of freedom. The resistive heater coil system enables thin aluminum specimens, held at the front-end of a heater resistant sabot, to be heate...

In engineering applications, materials are subjected to a wide range of conditions, which can be static or dynamic in nature, coupled with high levels of deformation and temperatures ranging from room to near the melting point. Under these thermomechanical extremes the material behavior can vary drastically; thus, over nearly a century, several experiments have been developed aimed towards probing the dynamic response and/or other characteristics of material behavior while under controlled loading regimes1,2,3,4,5,6,7,8,9,10,11,12,13,14. For metals loaded at low to intermediate strain rates (10-6-100 /s), servo-hydraulic or precision screw universal testing machines have been used to study the material response subjected to various loading modes and levels of deformation. But as the applied strain rates increase beyond the intermediate strain rates (i.e., &#62;102/s), other experimental techniques become necessary in order to probe the mechanical response. For example, at loading rates of 103/s up to 5 &#215; 104/s full-sized or miniaturized Split-Hopkinson pressure bars enable such measurements to be made8,15.
Traditionally, light gas-guns and/or explosively driven plate impact experiments have been utilized to study the dynamic inelasticity and other phenomenon such as spallation, or phase transformation that occur with very high strain rates (105-107/s)16,17,18,19,20,21,22, or combinations of high pressures and dynamic loading. Customarily, plate impact experiments involve the launch of a flyer plate carried by a sabot initially at the breech-end of the gas-gun, which then travels down the length of the gun-barrel and is made to collide with a carefully aligned stationary target plate at the impact chamber. As a result of the impact, normal and/or combined pressure and shear stresses are generated at the flyer/target interface, which travel through the spatial dimensions of the plates as longitudinal and/or combined longitudinal and transverse stress waves. The arrival of these waves at the rear surface of the target plate affect the instantaneous free surface particle velocity of the target plate, which is monitored typically via interferometric techniques. In order to allow the interpretation of the measured particle velocity versus time history, it is necessary that plane-waves with a front parallel to the impact surface be generated upon impact14,23. To ensure the former, impact must occur with an impact tilt angle on the order of less than one milli-radian12,24, with impact surfaces of flatness better than a couple micrometers5,25.
Plate impact experiments have been adapted to include heating elements which enable investigations of material behavior to extend into thermomechanical extremes26,27,28,29. These adaptations usually involve the addition of an induction coil, or of a resistive heater element to the target-end of the gas-gun; though these adaptations have been shown to be experimentally feasible, the approach inherently leads to special experimental challenges which require careful considerations. Some of these experimental complications include differential thermal expansion of the various constituents of the target holder assembly and/or alignment fixture while heating the target (sample) plate, which requires in-real-time alignment adjustments, usually made with remotely controlled alignment tools with continuous feedback in order to maintain crucial parallelism tolerance between the sample and target plate. In the case of the pressure-shear plate impact experimental scheme, heating the sample requires conventional polymer gratings be replaced by high-temperature resistant metallic gratings in order to monitor transverse particle velocity at the free surface of the target plate. Moreover, heating of the sample can add limitations on the impact velocity that can be employed in certain experimental schemes, such as in the high strain rate combined pressure-and-shear plate impact configuration, where special considerations may be required to prevent unambiguous interpretation of the experimental results, which are calculated using the acoustic impedance of the front and rear target plates which may be temperature dependent. Lastly, for other experimental schemes, which require a target plate with an optical window, tolerances between the sample, bond layer, and/or coatings become increasingly difficult to maintain at high temperatures19.
To alleviate the experimental challenges mentioned above, we have made custom adaptations to the existing single-stage gas-gun located at Case Western Reserve University (CWRU)7,30,31,32. These modifications enable thin metal specimens held at the front-end of a heat-resistant sabot to be heated to temperatures in excess of 1000 &#176;C, prior to firing, which allow high temperature normal and/or combined pressure-shear plate impact experiments to be conducted. In contrast to most of the conventional approaches employed for elevated temperature plate impact studies, this method has been shown to alleviate several of the experimental challenges described above. For example, this approach has been utilized to feasibly achieve tilt angles of less than one milli-radian without the need for remote tilt adjustment30, or additional optical elements for monitoring tilt changes during the experiment. Second, since the target plate remains under ambient temperatures, this method does not require the need for special high-temperature resistant holographic gratings for the measurement of transverse particle velocity in oblique impact experiments; additionally, higher impact velocities can be utilized without the risk of yielding the target plate, and thus, reduce the complexity in the interpretation of the experimental results. To add, this approach can be utilized to perform high temperature reverse-geometry normal plate impact experiments which provide Us-Up relationships for a choice sample material. These can be obtained via impedance matching techniques, or additionally, an analysis of the rarefaction fan from the back surface of the sample which carry information regarding changes in sample shock velocity during unloading33,34. In the elevated temperature combined pressure-shear plate impact configuration, this approach enables the dynamic inelasticity of thin films to be studied up to a wide temperature and plastic deformation range, and strain-rates up to 107/s depending of the thickness of the thin specimen16,27,29.
We will present the protocols necessary for performing a typical elevated temperature plate impact experiment discussed above. This will be followed by a section dedicated to representative results obtained using the present technique. Lastly, a discussion of the results will be presented prior to a conclusion.

<table>
	<tbody>
		<tr>
			<td>99.999% commercial purity polycrystalline aluminum</td>
			<td>Goodfellow</td>
			<td>AL007970</td>
			<td>Material for flyer plate (sample)</td>
		</tr>
		<tr>
			<td>H13 tool steel</td>
			<td>Fabrication Center of CWRU</td>
			<td>N/A</td>
			<td>Material for the sample holder</td>
		</tr>
		<tr>
			<td>Solution treat &#38; age Inconel 718 alloy</td>
			<td>High Temp Metals</td>
			<td>N/A</td>
			<td>(1.005/1.015)&#34; Dia x 24&#34;, Material for target plate</td>
		</tr>
		<tr>
			<td>Photoresist S1805</td>
			<td>MicroChem</td>
			<td>N/A</td>
			<td>Material of the photoresist for holographic grating</td>
		</tr>
		<tr>
			<td>Developer CD-26</td>
			<td>MicroChem</td>
			<td>N/A</td>
			<td>Developer to the photoresist for holographic grating</td>
		</tr>
		<tr>
			<td>Aluminum 6063 tube</td>
			<td>McMaster-Carr</td>
			<td>4568T19</td>
			<td>Material for the ring in target assembly</td>
		</tr>
		<tr>
			<td>Black Delrin (R) Acetal Resin Rod (4-1/2&#34; Dia.)</td>
			<td>McMaster-Carr</td>
			<td>8576K81</td>
			<td>Material for the Delrin holder in target assembly</td>
		</tr>
		<tr>
			<td>White Delrin (R) Acetal Resin Rod (1/4&#34; Dia.)</td>
			<td>McMaster-Carr</td>
			<td>8572K51</td>
			<td>Material for the Delrin pins in target assembly</td>
		</tr>
		<tr>
			<td>Aluminum 6061 tube</td>
			<td>McMaster-Carr</td>
			<td>9056K24</td>
			<td>Material for the body in projectile assembly</td>
		</tr>
		<tr>
			<td>Aluminum 6061 rod</td>
			<td>McMaster-Carr</td>
			<td>8974K88</td>
			<td>Material for the cap in projectile assembly</td>
		</tr>
		<tr>
			<td>Teflon sheet</td>
			<td>McMaster-Carr</td>
			<td>8711K98</td>
			<td>Material for the key</td>
		</tr>
		<tr>
			<td>LAVA-FF - Alumina Silicate disc</td>
			<td>Technical Products</td>
			<td>CWR-033116-1</td>
			<td></td>
		</tr>
		<tr>
			<td>LAVA-FF - Alumina Silicate tube</td>
			<td>Technical Products</td>
			<td>ALR11515</td>
			<td></td>
		</tr>
		<tr>
			<td>Alumina Pan Slotted Head Bolt</td>
			<td>Ceramco</td>
			<td>A83200PANSLT0.500</td>
			<td></td>
		</tr>
		<tr>
			<td>409 N70 Buna-N O-ring</td>
			<td>The O-ring Store</td>
			<td>B70409</td>
			<td></td>
		</tr>
		<tr>
			<td>Loctite Hysol 9412 adhesive</td>
			<td>Loctite</td>
			<td>83107</td>
			<td></td>
		</tr>
		<tr>
			<td>High Temperature Cements</td>
			<td>OMEGA Engineering</td>
			<td>OB-300</td>
			<td></td>
		</tr>
		<tr>
			<td>Extra fast-set epoxy</td>
			<td>Ellsworth</td>
			<td>4001</td>
			<td></td>
		</tr>
		<tr>
			<td>Mylar sheet</td>
			<td>McMaster-Carr</td>
			<td>8567K94</td>
			<td></td>
		</tr>
	</tbody>
</table>

conducting elevated temperature normal and combined pressure-shear plate impact experiments via a breech-end sabot heater system

A novel approach for conducting normal and/or combined pressure-shear plate impact experiments at test temperatures up to 1000 &#176;C is presented. The method enables elevated temperature plate-impact experiments aimed towards probing dynamic behavior of materials under thermomechanical extremes, while mitigating several special experimental challenges faced while performing similar experiments using the conventional plate impact approach. Custom adaptations are made to the breech-end of a single-stage gas-gun at Case Western Reserve University; these adaptations include a precision-machined extension piece made from SAE 4340 steel, which is strategically designed to mate the existing gun-barrel while providing a high tolerance match to the bore and keyway. The extension piece contains a vertical cylindrical heater-well, which houses a heater assembly. A resistive coil heater-head, capable of reaching temperatures of up 1200 &#176;C, is attached to a vertical stem with axial/rotational degrees of freedoms; this enables thin metal specimens held at the front-end of a heat-resistant sabot to be heated uniformly across the diameter to the desired test temperatures. By heating the flyer plate (in this case, the sample) at the breech-end of the gun-barrel instead of at the target-end, several critical experimental challenges can be averted. These include: 1) severe changes in the alignment of the target plate during heating due to the thermal expansion of the several constituents of the target holder assembly; 2) challenges that arise due to the diagnostics elements, (i.e., polymer holographic gratings, and optical probes) being too close to the heated target assembly; 3) challenges that arise for target plates with an optical window, where crucial tolerances between the sample, bond layer, and window become increasingly difficult to maintain at high temperatures; 4) in the case of combined compression-shear plate impact experiments, the need for high-temperature resistant diffraction gratings for the measurement of transverse particle velocity at the free surface of the target; and 5) limitations imposed on the impact velocity necessary for unambiguous interpretation of the measured free surface velocity versus time profile due to thermal softening and possibly yielding of the bounding target plates.By utilizing the adaptations mentioned above, we present results from a series of reverse geometry normal plate impact experiments on commercial purity aluminum at a range of sample temperatures. These experiments show decreasing particle velocities in the impacted state, which are indicative of material softening (decrease in post-yield flow stress) with increasing sample temperatures.

An 82.5 mm bore, 6 m length, single-stage gas gun at CWRU capable of accelerating 0.8 kg projectiles to speeds up to 700 m/s was used in conducting the present experiments. Figure 5 shows a photograph of the modified gas-gun facility at CWRU. Prior to firing, the custom designed sabot is housed within the heater extension piece, shown in Figure 6. The extension piece carries a vertical heater-well enabling a resistive coil heater...

Watch this Scientific Journal Video about Conducting Elevated Temperature Normal and Combined Pressure-Shear Plate Impact Experiments Via a Breech-end Sabot Heater System at JoVE.com

Conducting Elevated Temperature Normal and Combined Pressure-Shear Plate Impact Experiments Via a Breech-end Sabot Heater System

Here, we present a detailed protocol of a new approach for conducting elevated temperature reverse normal plate impact, and combined pressure-and-shear plate impact. The approach involves the use of a breech-end resistive coil heater to heat a sample held at the front-end of a heat-resistant sabot to the desired temperature.

A novel approach for conducting normal and/or combined pressure-shear plate impact experiments at test temperatures up to 1000 &#176;C is presented. The ...

A question of great interest is:How does the dynamic shearing resistance of metals respond to changes in pressure and applied strain rates? This important question in the dynamic material characterization field, and others, could be addressed by this technique which also mitigates several experimental variation sources that are present in the conventional approach. Though this method was was designed primarily for investigations of high temperature compressive and/or shearing resistance of materials, it can also be applied for studies such as dynamic slip and spall.The gas gun at Case Western Reserve University is single-stage, with a six meter length and 82.5 millimeter bore. One section houses a custom modification to the gun for this experiment. The modifications are represented on this schematic.A heating system mates with the existing gun barrel so that the sample held at the front of the custom heat resistant sabot can be heated to desired temperatures in excess of 1000 degrees Celsius. Additional modifications are at the target chamber end. There, a photonic doppler velocimeter probe precisely measures the velocity of the rear surface of the target plate.Trigger and tilt diagnostic signals come from the voltage-biased pins on the front surface of the target plate. The first step is to create the sample and target for the experiment. Create the samples using high purity polycrystalline aluminum.The equi-spaced holes are for securing the sample. Now check the surface flatness. Place the sample in contact with an optical flat then position the two under a green monochromatic light source.Observe light bands that are curved and spaced depending on the surface features. The surface is sufficiently flat if three or fewer light bands are visible across the sample diameter. The experiment also requires a target plate and aluminum ring.Make the target plates from precipitation hardened alloy rod. Section the ring from an aluminum tube and drill six equal-spaced slots in it before polishing it. Put the target plate and the aluminum ring together to produce an assembly similar to this.Next mount the assembly on the target holder ring. When done the assembly will include copper pins for connecting to the trigger and tilt diagnostic circuit. Mount the ring on the target holder.Gather the components of the heat resistant sabot. These include the sample holder, the sample, and the alumina screws. It also includes the alumina silicate lava rock tube and aluminum body tube with its cap.Now work with the cap. The cap has an eye bolt. It's grooves have a sealing O-ring and PTFE key.Run a thermo-couple wire through its bottom. Use epoxy to adhere the cap to the back of the aluminum tube. At the front of the aluminum tube epoxy the lava rock tube.Pull the thermocouple wire from the cap through the tubes to the sample holder and adhere the sample holder to the lava tube. After the cement dries secure the sample to the sample holder with the alumina screws. Before proceeding ensure that the flatness of the front surface of the sample has not changed.Now begin working with the gas gun at the impact chamber. Mount a three-axis motion stage on a rod above the gun barrel. Attach to the stage a prism holder with a precision optical alignment prism.Next tape a first surface mirror over the sample plate. Then position the sabot and the gun barrel with the sample facing out. At the target chamber put a first surface mirror on the target and put the target holder assembly in place.Carefully rotate the prism to be between the target and sample plates and their mirrors. Use a diffused bulb to perform a rough alignment of the plates. Adjust the stage to achieve a single continuous reflected image of the bulb from all surfaces of the prism.Go on to achieve fine alignment with the aid of an auto collimator. Tighten all the positioning screws on the target holder and remove the motion stage, prism, and mirrors. Move the sabot to the breech end of the gas gun and connect the thermocouple to the temperature monitor.At this point set up the doppler velocimeter. This requires an optical fiber focuser probe epoxied into an aluminum tube. Connect the optical focuser to an all-fiber-optic NDI TDI interferometer.Now take the focuser assembly to the target holder. This one is outside of the target chamber for demonstration. Use the attached amount at the rear to support the focuser.Aim the focuser at the rear surface of the target. Turn on the laser and adjust the focuser probe position using the screws on the focuser assembly. Stop after achieving proper light coupling and signal optimization.Adjust the variable ratio coupler to match the intensity of the reference and doppler shifted light to optimize the signal. After all preparations are complete seal the breech target chamber and evacuate the gun barrel to less than 100 millitorr. Move the heater into position and turn it on.Increase its temperature in 100 derees Celsius increments to reach the desired sample temperature. Pressurize the firing dump and load chambers. Also be sure to secure the sabot catcher to the impact chamber.With everything in place turn off the heater and immediately move it upwards toward the heater-well. Record the temperature of the thermocouple at the sample surface. From here continue with the normal firing sequence.This is an example of recorded voltage as a function of time during a typical reversed geometry normal plate impact experiment. The red trace is the trigger signal from the voltage based pins. The difference between the trigger times of the first and last pins allows an estimate of the maximum tilt at impact.A longitudinal wave arrives at the back surface. The black signal is the result of normal motion diagnostics of the free surface using the photon doppler velocimeter. This plot is of the free normal surface velocity for a range of temperatures.Data symbols go from black to red representing the coolest to warmest initial sample temperatures. The initial sharp rise is related to the stress of the aluminum sample at its interface with the target as the shock evolves. The plateau results from the impedance match between the sample and the target.Note that the highest free surface velocity is associated with the lowest temperature and velocities decrease with increasing temperature. This suggests possible thermal softening of the sample under the different test conditions. A more interesting result can be seen in figure 16 which shows the normal free surface particle velocity trace obtained from reverse geometry normal plate impact experiments performed on commercial purity polycrystalline magnesium.The results show monotonically decreasing particle velocities at the shock plateau with increasing temperatures in the range of 23 to 610 degrees Celsius. However at temperatures beyond this level, ie 617 and 630 degrees Celsius, a reversal of this trend can be clearly observed. This increase in particle velocity suggests an increase in the shock impedance of the sample material.Moreover assuming that the elastic constants of the material decrease as a function of increasing temperature then an increase in the shock impedance in this case suggests an increase in the yield strength and/or plastic modulus of the sample material. It can be seen that the increase in particle velocity at the shock plateau is accompanied by an increase in the particle velocity levels throughout the initial rise in the particle velocity trace, which correlates with the stress levels of the sample target interface during the incipient plasticity of the sample material. Figure 17 shows micrographs of cross sections of the impact surface of post-test specimens.The images show two noticeable effects on the microstructure as a result of increasing temperature. First is grain-ripening with increasing sample temperature. The images also show a change in twin band formations.Carefully looking at the images corresponding to temperatures ranging from 23 to 500 degrees Celsius, a clear reduction in twin bands are observed with increasing temperature. However, at higher temperatures, ie 610, 617, and 630 degrees Celsius, a reemergence of these twin bands are observed, which suggests that twin band formation is favored at the latter end of this temperature range. If the test materials are already assembled, once mastered, this technique can be done in about five to six hours.While attempting this procedure it's important to remember to take proper care in preparing the sample materials so that the flatness and parallelism tolerances can be adequately met. Following this procedure similar methods such as oblique plate impact and pressure shear plate impact can also be performed which enable dynamic material behavior to be probed at various loading rates and modes. After watching this video you should have a good understanding of how to perform an elevated temperature plate impact experiment which enables you to alleviate several experimental challenges present in the conventional approach.

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